Continuous monitoring of vital signs with the ability to remotely monitor patient status is a growing field and the ability to incorporate multiple measurement capabilities into a single small unobtrusive patch that can be worn by a patient (i.e., a body worn patch) for multiple days at a time is a desirable feature. One such vital sign measurement is a blood oxygen level reading, often carried out by a pulse oximeter.
Pulse oximeters typically shine light of two different wavelengths through a body part of a patient and measure relative differences in amplitude of the original light and the received light at the two different wavelengths. For example, one wavelength may be red light generated by a red light emitting diode (LED) and the other wavelength may be infrared light generated by an infrared LED. The relative differences in amplitude of the original red light and original infrared light and the received red light and infrared light may be measured by a phototransistor. Blood with lower levels of oxygen may tend to absorb less infrared light and more red light. Alternatively, blood with higher levels of oxygen may tend to absorb more infrared light and less red light. Thus, a properly calibrated pulse oximeter can determine blood oxygen levels by emitting light of red and infrared wavelengths and measuring the relative amounts of red and infrared light after the light passes through a body part of a patient, such as a fingertip or earlobe. Additionally, the heart rate for the patient can be determined by the pulse oximeter based on the measurement of the received light. The movement of the patient and/or improper placement of the pulse oximeter can result in degradation of the blood oxygen level measurements and heart rate measurements by a pulse oximeter.
The waveform of measured light received by the pulse oximeter, also referred to as the received signal, has a high direct current (DC) (i.e., relatively static) component and a comparatively lower alternating current (AC) (i.e., dynamic) component resulting from changes in blood flow, the most important cause of such changes being the patient's pulse. Both the AC and DC level of the received signal may vary greatly depending on the color of the patient's skin and/or the quality of the contact between the sensor elements of the pulse oximeter and the patient's skin. As examples, the quality of the contact between the sensor elements of the pulse oximeter and the patient's skin may be impacted by the positioning of the sensor elements, the state of the patient's skin (e.g., oily, dry, etc.), the user's hair, etc.
To help mitigate the impact of the varying AC and DC level of the received signal, a pulse oximeter may include an automatic gain control (AGC) loop in the control algorithm for the sensor elements. The AGC loop may amplify or reduce the voltage amplitude of the received signal to a level suitable for use by the measurement circuitry of the pulse oximeter, such as to within threshold input levels of an analog-to-digital converter (A/D converter).
In a similar manner, the waveform of the light transmitted by the pulse oximeter, also referred to herein as the transmitted signal, may be controlled by the AGC loop to amplify or reduce the intensity of light transmitted by the pulse oximeter to a level suitable for use by the measurement circuitry of the pulse oximeter. For example, the AGC loop may apply gain to the LED light pulses transmitted by the pulse oximeter into a patient to amplify or reduce the intensity of light transmitted to a level suitable for reception by the sensor elements of the pulse oximeter.